(1) Field of the Invention
This invention relates to a high precision electric motor which can be operated under the conditions of reduced noises, less vibrations and low cogging or low jitter and to a bearing system for use in such an electric motor. More particularly, the present invention relates to a compact spindle motor for use in the devices for, e.g., CD-ROM, HD, MO and the like in which devices high speed rotation without the reflection of shaft is required.
(2) Prior Art
In order to meet the above requirement, there has been proposed a method to form grooves in a bearing bore. Japanese Utility Model Publication No. 47-36739 discloses a bearing having dense and smooth protruded surfaces and recessed rough surfaces in combination in the bearing surface so as to reduce friction losses and noises. Japanese Laid-Open Utility Model Publication No. 61-101124 discloses a bearing which employs spiral grooves in the bearing bore intending the high rotational accuracy and the reduction of energy losses by introducing lubricant inside as the shaft turns. The bearing has additional advantages that the formation of grooves can be done without difficulty at low cost because the bearing is made of sintered powdered metal. Besides the above ones, Japanese Laid-Open Patent Publication No. 62-167921 and No. 62-167922 disclose a bearing in which the bearing bore comprises more than three plain surfaces formed by modifying the surfaces given by the circular arc, the curvature of which are larger than the that of the circular arc round the center of the bearing shaft, in combination. It is described that this bearing can reduce the friction losses due to the linear contact with the shaft. Japanese Laid-Open Patent Publication No. 5-115146 proposes an oil-impregnated sintered bearing having a plurality of substantially rectangular grooves so as to improve the hydrodynamic pressure functions, to restrain noises and to reduce wear with the simplified structure thereof.
As described above, in view of the increase in the fluid resistance of lubricant that is caused by the increase in revolution speed and the decrease in clearances, the above prior arts have intended to decrease fluid resistance by forming grooves in the bearing bore without expanding the clearance. At the same time, the grooves formed in the bearing bore improve the rigidity of the lubricant film at the shaft supporting potion due to the hydrodynamic pressure effect created in the groove area, thereby preventing the metal-to-contact and restraining the undesirable noises to occur.
The above method to form grooves in the bearing bore surface is indeed useful for reducing the friction loss, however, the accompanied hydrodynamic effect brings about a disadvantage that the vibration of shaft and bearing system is caused to occur. This is not preferable in view of the high precision revolution and it often leads to difficult troubles of making noises in the frequency range harsh to the ear and causing vibrations to occur.
Meanwhile, these bearing devices are mainly used for electric motors, so that, as one of elements for the motors, the relationship between the bearing device and the stator or the rotor of the motor is important. According to the number of the magnetic poles of the motor magnet and the number of the slots of the armature, the cogging or jitter is caused to occur with a harmonic function based on their least common multiples as a matter of their fundamental function, which invites the vibration of the motor itself, its media and the whole system. If the vibrations brought about by the number of magnetic poles and the number of the armature slots are harmonized with the vibrations brought about by the grooves of the bearing bore in frequency resonance condition, the vibrations are amplified with generating large noises and increasing the reflection of shaft, and thereby giving rise to the extreme local wear of the shaft and bearing system.